One measure to guard against spam is to close comments on old blog entries. This is easy enough to do.
At the bottom of your Edit Entry window is the pull-down for none, open, or closed comments. If you don't see this as pictured, click on "Customize the display of this page" link, select "Custom" and select the fields you wish displayed, including "Allow Comments".
You can also set comments to be open, closed or non-existent by default through the weblog config window, Preferences section. Under "Comment Configuration" see "Allow Comments" Default. Note that you can override this default via your selection on the individual entry.
There are several plugins that can be used to automatically close comments on old entries. See:
If you are using the MySQL database it is pretty easy to go in with CPANEL or a tool like CocoaMySQL for the Mac and edit the database to close comments on many entries at once.
"Comments are now closed" Message
If you do close comments, after they've been open and have accumulated a few, it helps to let your visitors know that comments are actually closed. Otherwise it can be a bit confusing; they see a link to comments, a list of comments, but no place to leave a comment.
To leave a "Comments are now closed" message that shows up when comments are closed, but not when they are open or non-existent, you can add the following code to the default MT Individual Entry Archive (assuming your comments are inline).
<p>Comments are now <b>closed</b> for this entry.</p><br />
Put this code right above the following tags in the MT3 default Individual Entry Archive template: